1. Field of the Invention
The present invention generally relates to a switching regulator, and more particularly relates to a switching regulator which switches a control status between a PWM control and a VFM control in accordance with a load of the switching regulator.
2. Description of the Related Art
Recently, electrical power saving in electronic devices has been demanded in terms of environmental problems. A trend such as this is especially prominent in battery operated electronic devices. In general, in order to save electrical power, it is important to reduce power consumption of the electronic device and to reduce wasteful power consumption in a power supply circuit by improving efficiency thereof. A non-insulated switching regulator including an inductor is widely applied as a high efficiency power supply circuit which is used in small electronic devices.
There are two main types known for controlling a switching regulator. The first is PWM (pulse width modulation) control which varies the duty cycle of a pulse signal in order to keep an output voltage of the switching regulator constant. The second is VFM (variable frequency modulation) control which varies the frequency of a pulse signal in order to keep an output voltage of the switching regulator constant.
According to the PWM control, a switching transistor included in the switching regulator is turned on/off at a constant frequency, even when an electrical load is relatively low. This causes a decrease in efficiency when the electrical load is relatively low. On the other hand, according to the VFM control, the frequency at which the switching transistor is turned on/off varies in accordance with the electrical load. Although the VFM control causes a larger effect of noise or ripple on the electronic device than the PWM control, the VFM control provides higher efficiency than the PWM control when the electrical load is relatively low. Because of these reasons, according to a conventional controlling method, the electronic efficiency is improved by switching the control status between the PWM control and the VFM control in accordance with the electrical load when the electrical load is relatively low or relatively high.
A general method for detecting an electrical load is by sensing a current flowing through a sensing resistor inserted into a current pathway between an input terminal and an output terminal. However, according to a method such as this, an electrical power loss at the sensing resistor increases as the current flowing through the sensing resistor increases. Thus, the method is not suitable for a battery operated small electronic device. There also is a method for detecting an electrical load indirectly by sensing a voltage of an error amplifier without using a sensing resistor (see, for example, patent document 1).
[Patent Document 1] Japanese Patent No. 3647811
The error amplifier includes an integrating circuit which eliminates ripple superimposed on the output voltage. The integrating circuit generally constitutes a phase compensation circuit. An output signal of the integrating circuit is at the same as time an output signal of the error amplifier. An integrating circuit such as this is optimized for a frequency of a PWM control. In a state where the switching transistor is turned on/off by a VFM control and the frequency for switching the switching transistor becomes lower than the frequency of the PWM control, the output signal of the integrating circuit is effective for sensing the electrical load (i.e. a load current) just after the switching transistor is turned on, but becomes ineffective while the switching transistor is turned off because the output signal becomes equal to the ground voltage or a voltage of a power source. The same can be applied in a state where the pulse of the PWM control is thinned out and the frequency for switching the switching transistor becomes lower. It becomes difficult to keep an output voltage of the error amplifier constant against the load current, and thus the relationship between the output voltage of the error amplifier and the load current becomes nonconstant. Thus, it is difficult to set the load current accurately when switching the control status between the PWM control and the VFM control by using an error amplifier compared with switching the control status by using an sensing resistor.